The present invention pertains generally to laser systems. More particularly, the present invention pertains to laser beam delivery systems for simultaneously creating a plurality of focal points from a single laser source. The present invention is particularly, but not exclusively, useful for creating a plurality of laser focal points suitable for performing refractive surgery.
In a conventional LASIK procedure, a corneal flap is first created with a microkeratome. Next, the flap is lifted to expose stromal tissue. Once exposed, the stromal tissue is photoablated using an Excimer laser. After photoablation, the flap is replaced over the cornea and the cornea is allowed to heal. The result is a reshaped cornea. By reshaping the cornea in this manner, vision deficiencies in the patient can be corrected. There are, however, several drawbacks associated with using a microkeratome to create the flap. For one, using a microkeratome is labor intensive. Additionally, the results obtained when using a microkeratome are highly dependent on the skill of the surgeon. Finally, shape of the exposed bed of stromal tissue that results from the use of a microkeratome is generally limited to flat surfaces. Because of these drawbacks, the development of new techniques for creating corneal flaps is merited.
One technique for creating a corneal flap that is gaining widespread acceptance involves the use of a pulsed laser beam to photoalter stromal tissue. In this technique, a pulsed laser is focused beneath the anterior surface of the cornea to a focal point within the stroma. For example, a pulsed laser beam having a pulse frequency of approximately 4 kHz with pulse durations as long as a few nanoseconds or as short as only a few femtoseconds can be used for subsurface photoalteration of stromal tissue. For a commonly used focal point size of approximately 10 xcexcm in diameter, a typical laser source can produce an average pulse energy of approximately 60 xcexcJ at the focal point. This energy (60 xcexcJ) is far in excess of the energy required to photoalter stromal tissue. Specifically, only about 2 xcexcJ is required for photoalteration of stromal tissue, with about 5 xcexcJ being optimal. Consequently, when only a single focal point is used, most of the energy available in a typical pulsed laser source is wasted.
Consider now an exemplary flap for a LASIK procedure having a diameter of approximately 10 mm. For this flap, the photoalteration of approximately 200,000 stromal points is required. Stated another way, approximately 200,000 pulses, with each pulse having an average energy of approximately 5 xcexcJ, are required. Continuing with this example, for a 4 kHz laser using a single focal point, about 50 seconds would be required to create a 10 mm flap. It is to be appreciated that procedures requiring this length of time (i.e. 50 seconds), pose a number of serious problems. One problem with lengthy procedures is eye movement. To overcome eye movement, eye restraint is often used. Unfortunately, restraining the eye for long periods of time can cause discomfort for the patient. Another problem associated with long procedure times involves patient blinking. Each time a patient blinks, a new tear film is deposited on the anterior surface of the cornea. Each tear film affects the optical path of the laser beam in a slightly different manner, affecting the precision of the operation. Thus, it is preferable to perform an entire procedure with a single tear film, if possible. Typically, 10 seconds is about the maximum time that a patient can restrain from blinking, thus it is preferable to complete an entire procedure in less than about 10 seconds.
In all surgical procedures, damage to non-target (i.e. collateral) tissue is to be avoided. During photoalteration of target tissue, nearby (non-target) tissue is heated. Some heating of non-target tissue can be accommodated without damage to the non-target tissue. Specifically, for stromal tissue, a temperature rise of about 3xc2x0 C. can be tolerated without long-term cell damage. In contrast, temperature increases of between about 8xc2x0 C. and 23xc2x0 C. can result in tissue shrinkage, cell denaturation, loss of cell function and coagulation. Importantly for the present invention, when multiple focal points are used to simultaneously photoalter tissue, a minimum spacing between adjacent focal points is required to prevent damage to non-target tissue from the heat generated during photoalteration.
In light of the above, it is an object of the present invention to provide a laser system suitable for the purposes of expeditiously photoaltering stromal tissue without heating collateral tissue to harmful temperatures. It is another object of the present invention to provide a pulsed laser system capable of generating a plurality of spaced apart laser focal points, with each focal point having a suitable pulse energy to accomplish photoalteration of stromal tissue. It is yet another object of the present invention to provide a laser system that partitions a pulsed laser beam into a plurality of laser focal points having adequate spacing between focal points to allow the heat generated during photoalteration to dissipate, thereby preventing heat damage to non-target tissue. Still another object of the present invention is to provide a multiple focal point pulsed laser system capable of photoaltering approximately 200,000 points within the stroma in less than approximately 10 seconds. It is still another object of the present invention to provide a multiple focal point, pulsed laser system capable of expeditiously photoaltering an entire 10 mm corneal flap during the period between the blinks of the patient (i.e. in less than approximately 10 seconds). Yet another object of the present invention is to provide a laser system and a method for its use which are relatively easy to use, simple to implement, and comparatively cost effective.
The present invention is directed to an optical system for partitioning and focusing a laser beam onto a plurality of focal points to simultaneously photoalter corneal tissue at a plurality of locations. The plurality of focal points can be scanned, as a group, through the cornea to quickly and safely photoalter a predetermined volume of subsurface corneal tissue. For the present invention, the system includes a laser source capable of generating a pulsed laser beam (hereinafter referred to as a master beam) having a pulse frequency of approximately 4 kHz and an average pulse energy of approximately 60 xcexcJ.
In one embodiment for the present invention, the master beam produced by the laser source is directed into a lenslet array to partition the master beam into a plurality of beams. Preferably, the lenslet array has six lenslets arranged in a circle surrounding a center lenslet. Thus, seven spaced apart beams emerge from the lenslet array. From the lenslet array, the seven beams are directed into a series of optical lenses and a scanner. In detail, the seven beams are first directed into a field lens to diverge the seven beams. From the field lens, the diverging beams are directed to a collimating lens to place the seven beams onto parallel beam paths. Next, the collimated beams are directed to a pair of relay lenses arranged as a telescope to magnify the collimated beams.
Once magnified, the beams are directed to a cutting lens to focus each of the beams to a separate focal point. Thus, a group (or cluster) of focal points is established. Like the lenslet array, the cluster of focal points is preferably arranged with six focal points distributed uniformly around a circle with the seventh focal point positioned at the center of the circle. For the present invention, a scanner is provided to move the cluster of focal points, as a group, through the cornea. Preferably, the scanner is interposed between the relay lenses that magnify the collimated beams.
In another embodiment for the present invention, an active mirror having approximately 40,000 active facets can be used to partition the master beam into seven diverging beams. The diverging beams from the active mirror are then collimated, magnified and focused using the optics described above. For both embodiments, a plurality of focal points suitable for subsurface photoalteration of corneal tissue is obtained, with each focal point having an average pulse energy of approximately 5 xcexcJ.